Gases, along with liquids and solids, are one of the states of matter. Specifically, very low density, compressible (which means changes volume when pressure increases, as opposed to liquids which are generally incompressible). Gases are fluids, which means that they flow easily. Most materials will turn into a gas if they get to a high enough temperature.
The University of Colorado has graciously allowed us to use the following Phet simulation. Explore the simulation to see how states of matter change depending on temperature and pressure: